Measuring apparatus



April 6, 1943- H. s. JONES 2,315,714

MEASURING APPARATUS Filed June 12, 1942 l4 l7 l8 HG. 2. INVENTOR.

HARRY S. JONES 63 AT RNEY.

Patented Apr. 6, 1943 MEASURING APPARATUS Harry S. Jones, Washington, D. 0., assignor to The Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Application June 12, 1942, Serial No. 446,831

31 Claims.

The present invention relates to apparatus having especial utility in measuring the magnitude and changes in magnitude of a variable condition, and more specifically, to self balancing electrical apparatus having particular utility in measuring the magnitude of electrical, thermal, chemical, physical, and mechanical quantities or qualities such as electric current, temperature, pressure, flow, or hydrogen ion content. The present invention is particularly useful in pyrometric apparatus for measuring voltage changes in thermocouples which are exposed to variations in temperature or radiant energy. The invention is also useful in many other different and varied applications among which may be included measuring electrolytic conditions of industrial cells and in particular the determination of hydrogen ion content values of cells wherein it is desired to measure small voltage variations accurately and automatically.

A general object of the invention is to provide in measuring apparatus of the type referred to above means for automatically compensating for the disturbing influence ofexternal factors on the accuracy of the measurement obtained.

A specific object of the invention is to provide in measuring apparatus of the type referred to above means for automatically compensating for the disturbing influence of external factors such as variations in the ambient temperature to which the apparatus is subjected and variations in voltage of the energizing source which is utilized for supplying electrical current to the apparatus.

Another specific object of the invention is to provide measuring apparatus of the pyrometric type utilizing a thermoelectric element having an improved means for compensating for variations in temperature in the cold junction of the thermoelectric element so that accurate measurements may be obtained at all times over a wide range of cold junction temperatures.

A further specific object of the invention is to provide a pyrometer potentiometer having an improved means for compensating for variations in temperature of the cold junctions of the thermocouple. v

A still further specific object of the invention i; to provide a pyrometer potentiometer having improved means for compensating for variations in temperature of the cold junctions of the thermocouple and also incorporating means for compensating for variations ,in the voltage of the electric energizing source employed to energize the potentiometer.

The foregoing objects are obtained in accordance with the present invention by an arrangement wherein a resistance bridge is energized from an alternating current source through a suitable rectifier and is connected in circuit with a thermocouple to comprise a system of the potentiometer type. The resistance bridge includes suitable temperature responsive means in certain of its arms whereby the bridge is unbalanced upon variation in the ambient temperature to which the temperature responsive resistances are subjected. The cold junctions of the thermocouple and the temperature responsive resistances of the bridge are subjected to the same ambient temperature variations, and as a result, the unbalance produced in the bridge upon'variation in ambient temperature is a function ofv the change in thermocouple electromotive force produced upon the variations in ambient temperature. By suitably choosing the resistance elements of the bridge the unbalance produced in the bridge upon variation in ambient tempera ture may be related to the change in thermocouple electromotive force produced upon variation in ambient temperature as is required to exactly compensate for changes in ambient temperature variations.

In accordance with the present invention the bridge network may also be used to provide compensation for variations in the known electromotive force which is opposed to the thermocouple electromotive force in the potentiometer circuit. This advantageous result may be produced when the known electromotive force is derived from the same source of electrical current which is utilized to energize the compensating bridge circuit. When the resistance elements of the bridge circuit are properly chosen in relation to each other, the current flows through certain of the bridge arms may bear a straight line relationship to the voltage drop across those bridge arms while no such straight line relationship obtains for certain other of the bridge arms. Consequently, as the energizing current flows to the bridge changes, the voltage drop across certain of the bridge arms changes to a different extent than the voltage drop across other of the bridge arms to thereby effect a change in the state of balance of the bridge. This change in the unbalance of the bridge may be utilized, as will be recognized by those skilled in the art, to compensate for the variations in the known electromotive force caused by such changes in the voltage of the energizing current source. Such compensation may be effected to a relatively high degree of accuracy by properly proportioning and designing the elements of the bridge network.

The various features of novelty which characterize the present invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages, and specific objects attained with its use, reference should be had to the accompanying drawing and descriptive matterin which I have illustrated and described a preferred embodiment of the invention.

Of the drawing:

Fig. 1 is a diagrammatic representation of one embodiment of the invention; and

Figs. 2 and 3 illustrate in detail one form of converter which may be utilized in the arrangement of Fig. 1.

Referring now to Fig. 1 of the drawing there is illustrated in schematic form an arrangement including an electronic amplifier 1, shown in detail, for producing effects in accordance with the extent of unbalance of a potentiometer network which controls the electronic ampiifler l and is unbalanced in accordance with the variations in a quantity to be measured, and in which because of the small magnitude of the unbalanced potentiometer electromotive forces, it is not practicable, nor desirable, to have the elfects produced directly by the potentiometric network.

More specifically, an arrangement is illustrated in Fig. 1 for indicating and recording the temperature within a furnace 2 in the interior of which a thermocouple 3 is arranged so as to be responsive to slight changes in temperature is of the type shown and illustrated in application Serial Number 421,176 and filed by Frederick W. Bide on December 1, 1941. The interrupter I is provided with a base 13 in which are mounted terminals I4, 15, l8, l1, l3 and I9. A plate 29 is screwed to the base I3 by means of screws 2|. A stud 22 provided with a screw threaded extension 23 is secured to the lower end of the plate 25 by means of a lock washer 24 and a nut 25. The free end of the stud 23 is bifurcated having spaced apart ends 25 and 21. Located between the spaced ends 26 and 21 are an insulating pad (not shown), a spring contact arm 28 carrying a contact 29, a resilient stop 30, an insulating pad 3|, a vibrating reed 32 carrying a contact 33, an insulating pad 34, a resilient stop 35, a spring contact arm 36 carrying a contact 31, and an insulating pad (not shown). These elements are all clamped between the spaced apart ends 25 and 21 by a bolt 38 and a nut 39. The spring contact arms 29 and 36 are provided with cars 40 and 4| respectively, which are electrically connected to the terminals l5 and I6 respectively. The vibrating reed 32 is provided with an ear 42 which is electrically connected to the terminal l4. Riveted to the supporting plate 20 are also studs 43 and 44 which carry adjustable stops in the form of screws 45-,and 46 formed of insulating material. When the adjustable stops 45 and 46 have been adjusted as desired they are clamped in place by means of screws 41 and 45 cated at a distance from the remainder of the measuring apparatus, is connected by a pair of conductors 4 and 5 into a potentiometric measuring circuit indicated generally by the reference numerali. The potentiometric network also has connected therein a fixed resistance 1, an interrupter or converter device 8, and the primary winding 9 ofa transformer l0 having a secondary winding II. The interrupter or converting device 8 is described in detail in connection with Figs. 2 and 3.

In the arrangement-of Fig. 1 the electromotive force developed by the thermocouple 3 is opposed to the potential drop produced across, the fixed resistance 1 by a flow of direct current therethrough. The electric current flow through the resistance 1 is derived from the output circuit of the amplifier i by means of a reconverter H which operates to transform the alternating current output of the amplifier i into a corresponding direct current. The output circuit of the amplifier l is controlled in accordance with the state of balance between the opposed electromotive forces in the potentiometric network 6, and therefore, the current flow through the resistance 1 is also controlled in accordance with the state of balance of those electromotive forces.

The interrupter 8, illustrated schematically in Fig. 1 and in greater detail in Figs. 2 and 3, operates to convert the potentiometric unbalance direct currents into pulsating currents which are capable of being readily amplified. It will be understood that any suitable current interrupter may be employed for this purpose, but in order to illustrate an operative embodiment, the interrupter shown in detail in Figs. 2 and 3 may be utilized.

The interrupter l illustrated in Figs. 2 and 3 respectively. The spring contact arm 36 carrying the contact 31 through its own resiliency engages the resilient stop 35 and the resilient stop 35 through its ownresiliency engages the adjustable stop 45. In like manner the spring contact arm 28 engages the resilient stop 39 which in turn engages the adjustable stop 36. By adjustingthe adjustable stops and 48 the positions of the contacts 31 and 29 may be independently adjusted with respect to the contact 33 carried 'by the vibrating reed 32.

A permanent magnet 49 is secured to the supporting plate 20 by screws 50 and 5|. A coil 52 is held in place by a bracket 53 which in turn is secured in place by the screws 59 and 51. The end of the vibrating reed 32 is disposed within the coil 52 and is provided with an armature 54 which is riveted to the vibrating reed 32 by rivets 55 as seen in Fig. 3.

The coil 52 is energized with alternating current and acts on the armature 54 to vibrate the reed 32 at cycles per second, when the alternating current supplied by the alternating current supply mains L and L is 60 cycle alternating current, to cause the contact 33 to engage and disengage with contacts 31 and 29 at the same frequency. The permanent magnet 49 operates in conjunction with the coil 52 and the armature 54 in such a manner as to cause the armature 54 to vibrate in synchronism with the alternating current supplied by the mains L and L By adjusting the adjustable stops 45 and 4t and hence the contacts 31 and 29 the wave form produced by the contacts 29, 33 and 31 may be adjusted to the desired value and shape. The

contacts 29 and 31 are preferably so arranged that when the contact 33 is in its stationary position it engages both contacts 31 and 29. This provides an overlapping action which compensates for wear of the contacts and also contributes to the elimination of stray electrical effects on the operation of the apparatus. Due

to this overlapping action also wear of the contacts does not materially alter the wave form produced bythe contacts. By mounting the contacts 24 and 31 on the spring contact, arms 24 and 88, respectively good wiping contact is at all times provided by the contact 33 and the contacts 29 and 31. An electrical connection may desirably be provided between one of the screws II and ground .so that the various parts of the interrupter may be connected to ground to maintain the interrupter at ground potential. A cover (not shown) may also desirably be provided for enclosing the movable parts of the interrupter. Such a cover may be held in place on the base II by means of a rolled flange clamping the cover to the base. Such a cover will act to prevent dirt and corrosive atmosphere from affecting the parts ofthe interrupter.

The interrupter 8 is essentially a polarizing switching mechanism, the operating winding 52 and the permanent magnet 49 cooperating to vibrate the reed 32 at 60 cycles per second insynchronism with the 60 cycle alternating current supply. For purposes of explanation it may be assumed that the contact 31 is engaged by the contact 33 during the first half cycle of the alternating current supply when the voltage is positive and the second contact 29 is engaged by the contact as during the second half cycle when the alternating voltage supply is negative, Accordingly, the contacts 33 and .31 engage fw'hen the voltage of the alternating current supply is positive and the contacts and 28 engage when the voltage of the alternating current supply is negative. when the vibrating reed 32 is stationary in its mid position both contacts 2s and 31 will he engaged by the contact 33 so that when the vibrating reed is operated the contact 33 is always in engagement with one or the other of the contacts 29 and 37!.

The manner of connection or the interrupter B to the primary winding 9 of the transformeriil is schematically shown in Fig. 1. By reference to Fig. 1 it will be noted that the contact 29 is connected to one end terminal of the transformer primary winding 9 and the contact 37 is connected to the other end terminal of the primary winding 9. The contact 33 is connected through the vibrating reed 32 and the terminal I4 to the conductor 6 and thereby to one terminal of the potentiometric circuit 6, and the center tap on the transformer primary winding 9 is connected by a conductor 56 to the other terminal of the poteniometric circuit. I

When the potentiometric network 6 is balanced,

- no current flows in the potentiometric network,

and accordingly, operation of th interrupter Q is ineffective. When the electromotive force developed by the thermocouple 3 increases to unba ance the potentiometric network in one direction, however, the unbalanced potentiometric current flows in the direction from the transformer pri mary winding 9 to the interrupter 8, and conversely, when the electromotive force developed by the thermocouple 3 decreases to unbalance the potentiometric network 6 in the opposite direct-icn, the potentiometric unbalanced currents flow in the direction from the interrupter B to the transformer primary winding 9.

- .Morespeciflcally, upon unbalance of the potentiometric network in one direction to render the conductor 56 positive with respect to the conductor 4, unbalanced current during the first half cycle of the alternating current supplied by the mains L and L will flow from the conductor 56 to the midpoint of the primary winding 9 through the lower half of the winding 9 to the contact 31,

to the contact 38, and the reed 82 to the conductor 4. During the second half cycle of the alternating current supply current will flow from the conductor 58 to the midpoint of the transformer primary winding 9 through the upper half of the winding 8 to contact 29, to contact 33 and through the vibrating reed 32 to the conductor 4. The pulsating direct current flows through the transformer primary winding 9 first through the lower half of the winding and then through the upper half of the winding acts through the core structure of the transformer Ill to induce an alternating voltage in the secondary winding II having a predetermined phase relatively to the phase of the alternating current supplied by the mains L and L.

Upon unbalance of'the potentiometric network in the opposite direction, the conductor 4 will be rendered positive relatively to the conductor 56, and therefore, the current flows through the transformer primary winding 9 will alternately be from the lower end of the winding to the midpoint when the contacts 33 and 31 are in engagement, and from the upper end of the winding to the midpoint when th contacts 29 and 33 are in engagement. These pulsating direct currents through the transformer primary winding 9 are in the opposite direction from what they were when the potentiometric network was unbalanced in the opposite direction, and as a result, these pulsating direct current flows operate through the core structure of the transformer it to induce a voltage in the transformer secondary windin H which is of the opposite phase relatively to the voltage of the alternating current supply mains is and Li.

Accordingly, when the potentiometric network 8 is unbalanced in one direction, an alternating voltage of one phase relative to the voltage of the supply mains L and L is induced in the trans former secondary winding H, and when the potentiometric network is unbalanced in the opposite direction an alternating voltage of the opposite phase relative to the voltageof the supply mains L and L is induced in the transformer secondary winding ii. When the network is precisely balanced there is no current flow through the primary winding 9 of the transformer ii and hence, the alternating voltage derived in th transformer secondary winding Ii is zero.

When the electromotive force developed by the thermocouple is exactly that value which is maintained across resistance 1 by the flow of current therethrough from the output circuit ofthe reconverter iii. he differentia between the opp sed potentials will be zero, and therefore, the alternating current inducedin the transformer winding H and impressed on the input circuit of the amplifier H will also be zero. .Under this condition the direct current output from the reconverter 12 will be that value needed to produce a potential drop across resistance 1 of the required magnitude .to balance the thermocoup e electromotive force. In this condition of operation of the measuring apparatus the potentiometric notwo k 6 is precisely balanced.

Upon a change in the electromotive force developed by the thermocouple 3 the state of precise balance of the potentiometric network 6 will be disturbed and a difference in potential between the two opposed potentials will be produced. This difference in potential is translated by the interrupter 8 and the transformer iii into an alternating current of one phase or of opposite phase depending upon the polarity of that difference in potential and thereby upon the direction of the change in the thermocouple electromotive force. This alternating current is amplified by the electronic amplifier l and operates to vary the direct current output from the reconverter |2 as is required to reduce the difference in potential between the opposed electromotive forces in the potentiometric network. That is to say, the changes in the output'current from the reconverter l2 operates to produce a corresponding change in potential drop across resistance 1 which is in the proper direction to tend to restore the state of balanceof the potentiometric network 6. The change in output current from reconverter i2 required to restore the network 6 to its balance condition depends upon the extent to which the electromotive force of thermocouple 3 has changed.

As will be recognized by those'skilled in the art, the potentiometric network 6 will not be returned to its exact condition of balance since some slight unbalance of the potentiometric network 6 is required to maintain the new value of output current from the reconverter i2. The degree of unbalance of the potentiometric network required to maintain this new reconverter output current is insignificant, however, and therefore,

for all practical purposes the potentiometric network 6 is 'rebalanced.

A mllliammeter recorder 61 and a milliammeter indicator 58 are connected in series -with the resistance 1 in the output circuit of the reconverter 2 and are provided to measure the current flowing through the resistance 1 from the reconverter l2. It will be recognized that the current flow through the resistance 1 required to balance or stabilize the electromotive force developed by the thermocouple 6 will provide a measure of the temperature to which the thermocouple is subjected. Thus, the milliammeter instruments provide a record or indication of the temperature within the interior of the furnace to which the thermocouple is subjected.

The electronic device I includes an electronic tube 66 to the input circuit of which the alternating voltage induced in the transformer secondary winding II is applied. The transformer secondary winding may be shunted by a condenser 66 of suitable value for tuning the winding ii to obtain the most efiicient and satisfactory operation. The electronic tube 66 includes two heater type triodes, which have been designated by the reference numerals 6| and 62, within the same envelope. The triode 6| includes anode, control electrode, cathode and heater filament elements, and the triode 62 includes like elements. The filaments of the triodes 6| and 62 may be connected in parallel and are arranged to receive energizing current from the lowvoltage secondary winding "of a transformer 66 having a line voltage primary winding 66, low voltage secondary windings 66, 61 and 66, and high voltage secondary windings 66, I6 and 1| and I2. The conductors to theheater filaments of the electronic tube 66 have not been shown in order to avoid complication of the drawing. The primary winding 66 of the transformer 64 is connected to and receives energizing current from the alternating current supply mains L and 13.

As illustrated, a portion of the transformer secondary winding 66 is connected to and supplies energizing current to the operating coil 62 of the interrupter 6.:

The transformer secondary winding 66 is also connected by means of conductors (not shown) to the heater filaments of an electronic valve 16. The electronic valve 16 includes two heater type triodes, designated by the reference characters H and I6, within the same envelope. Both of the triodes of valve I6 include anode, control electrode, cathode, and heater filament elements.

The triode II of the electronic tube I6 is utilized as a half-wave rectifier to supply a source of direct current voltage for supplying the output circuits of the triodes 6| and 62 and I4. The control electrode and cathode of triode 16 are connected to each other and the output circuit thereof is energized by the transformer secondary winding 66 through a circuit which may be traced from the right end terminal of the winding 66, as seen in the drawing. through a conductor l6 to the anode of the triode I6, the cathode thereof, and through a conductor 11 to the positive terminal. of a filter which is designated by the reference numeral 16. The negative terminal of the filter 16 is connected by a conductor 16 to the l'eftend terminal of the transformer secondary winding 66.

The filter 76 includes a condenser 66 which operates to smooth out the ripple in the output voltage of the filter between the points 6| and 62. The filter 16 also includes a resistance 66 and a condenser 66 which operates to smooth out the output voltage of the filter between the filter points 66 and 62. The filter 16 further includes a resistance 66 and a condenser 61 for smoothing out the output voltage between the filter points 86 and 62. The filter, therefore, comprises 6 stages. A three stage filter is provided because for the most efficient and satisfactory operation,

it is necessary that the anode voltage supply to the triode 66 be substantially free from ripple whereas it is not necessary to supply anode voltage so free from ripple to triode 62. In addition, it is not necessary to supply anode voltage as free from ripple to the triode H as it is to the triode 62.

The anode circuit of the triode 6| may be traced from the filter point 66 which comprises the positive terminal of the filter through a fixed resistance 66 to the anode of the triode 6|, to the oathode thereof and through a cathode biasing resistance 66 which is shunted by a condenser II, to the negative filter point 62 through a conductor 62. The cathode biasing resistance 66 and the parallel connected condenser 6| are utilized for biasing the control electrode of the triode 6| negatively with respect to the cathode.

The input circuit of the triode 6| may be traced from the cathode to the parallel connected resistance 66 and condenser 6| through the transformer secondary winding II to the control electrode of the triode 6|.

The output circuit of the triode 6| is coupled by means of a condenser 66 and a resistance 64 to. the input circuit of the triode 62. More particularly, the anode of the triode 6| is connected by condenser 66 to the control electrode of the triode 62 and the control electrode of the triode 62 is connected through the resistance 66 to the conductor 62 and therethrough to the cathode of the triode 62.

The anode circuit of the triode 62 may be traced from the positive terminal 66 of the filter 16 through a fixed resistance 66 to the anode of the triode 62, the cathode thereof, a biasing resistance 66 which is shunted by a condenser 61, and conductor 62 to the negative terminal 62 of the filter.

The output circuit of the triode 62 is resistance capacity coupled by means of a condenser 98 and a resistance 99 to the input circuit of the trlode 14. As illustrated, a contact I99 which is in engagement with the resistance 99 is provided for varying the point of contact of the control electrode of the trlode 14 to the resistance 99. The

resistance 99 and contact I99 perform a dual ftmction, namely, to limit the extent to which the trlode 14 may be driven positive relatively to its associated cathode and also to vary the signal 19 impressed upon the control electrode from the output circuit of the trlode 92. It is noted the resistances 94 and 99 which are connected in the input circuit of the triodes 92 and 14, respectively operate to maintain the control electrodes of the 15 triodes 92 and 18 at ground potential when no voltage is induced in the transformer secondary winding II', as is also the'control electrode of triode 9|. Upon the induction of an alternating voltage in the secondary winding II, resistances go 98 and 99 permit the flow of currentbetween the control electrodes of the triodes 92 and 14 and their associated cathodes to thereby limit the extentto whichthe control electrodes of the triodes 92 and 14 are permitted to go positive with respect to their associated cathodes.

Anode voltage is supplied the output circuit of the trlode 14 from the filter 18 through a circuit which may be traced from the filter point 9| through a fixed resistance IN, the anode of the trlode 14, the anode to cathode resistance, and

conductor 92 to the negative filter point 82.

The output circuit of the trlode 14 is coupled by means of a condenser I92 and a transformer I99-to the input circuit of an electronic valve I94.

transformer I93 has its terminals connected to i the input circuit tube I94.

4 The electronic tube I94 includes two heater type triodes which have been designated by the reference numerals I98 and I99 within the same envelope. Both of the triodes includes anode,

control electrode, cathode, and heater filament elements. The heater filaments of the triodes I98 and I99 may desirably be connected in parallel with each other and receive energizing current through conductors (not shown) from the transformersecondary winding 98.

The input circuits of the triodes I98 and I99 I are connected in parallel to th transformer secondary winding I91. Specifically, one terminal of the transformer secondary winding I91 is connected to both of the control electrodes of the triodes I98 and I99, and the other terminal of the transformer secondary winding I91 is connected to the cathodes of the triodes I98 and I 99.

Anode voltage is supplied to the output circuits of the triodes I98 and I99 from th high 6. voltage secondary windings 19 and 1 I. Thus, the

anode of the trlode I98 is connected to the rightend terminal of the transformer secondary winding" and the anode of the trlode, I99 is connected to the left end terminal of the transformer 7 secondary winding H. The anode circuit of the trlode I98 may be traced from the right end terminal of the winding 19 to the anod of trlode I98, the cathode thereof, a conductor H9, and a conductor III, in which a resistance II2 shunted by a condenser I I9 is inserted, to the left end terminal of the winding 19. The anode circuit of trlode I99 may be tracedfrom the left end of the winding H to the anode of triodeI99, to the oathode thereof, conductor I I9, and a conductor I I3a in which a resistance I ll shunted bya condenser H5 is inserted. to the right end terminal of the winding 1 I. The transformer secondary windings 19 and H are so wound on the transformer 64 that the triodes I98 and I99 are adapted to be rendered alternately conductive, that is, one triode will be conductive while the other is non-conductive and vice versa.

When the signal impressed on the input circuits of triodes I98 and I99 by the transformer secondary winding I91 is zero, the triodes I98 and I99 will be equally conductive during alternate half cycles, and consequently, the average current flow through the resistance I I 2- will be the same as that through the resistance IIQ. The current fiows through the resistances H2 and II4 will be in opposite directions, however, and therefore; the

.potential drop acrossresistance I I2 willbe opposed to that across resistance I. When resistances I I 9 and II! are equal'in magnitude the potential drop so produced across these resistanoes will then be equal in magnitude.

The resistances H2 and Ill are connected in the input circuit of an electronic valve I19 and i the resultant potential drop across these resistances is utilized for the purpose of controlling the conductivity of valve H9. The valve II9 may be of the type known commonly as beam power am plifier valves, and includes an anode, a' control electrode, a screen electrode, a beam forming plate, a cathode, and a heater filament. The heater filament may desirably be connected in parallel with the heater filaments of triodes I98 and I99 to the terminals of the transformer secondary winding 98 for'receiving energizing current from the latter. The input circuit of the valve I I9 may be traced from its cathode through the resistances II! and H2 and through a current limiting resistance I I! to the control electrode of valve I I9.

.Anode voltage is suppliedto the electronic valve II9 from the full wave rectifier arrangement including an electronic valve H8 and a filter II 9. Valve I I8 includes a filament type cathode and a pair of anodes. The filament cathode is connected to and receives energizing current from the transformer secondary winding 91. One anode of the valve H8 is connected toone terminal of the transformer secondary. winding 12 and the other anode of the valve H8 is connected to the other terminal of the winding 12. The rectified current from the valve I I8 is connected to the input terminals of filter H9, which may be of any suitable ty-pe, through a circuit which may be traced from the filamentof valve II8 to one in- .put terminal of filter I I9 and from the other input terminal of the filter to the center tap of transformer secondary winding 12. The filter I I9 includes suitable capacative and inductive elements and has its positive output terminal directly connected to the anode of the valve H6. The output circuit of the valve II6 may be traced from the positive output terminal of the filter II9 to the anode of valve II9, the cathode thereof, the

fixed resistance 1, the milliammeter recorder 51 and the milliammeter indicator 58 to the negative output terminal of the filter I I9.

When the potential drop across the resistance III is equal to that across the resistance II2, the potential of the control electrode of valve IIS will be the same as the potential of the cathode thereof and under this condition the current flow in the output circuit of valve I I6 will be a prede termined value. The fixed resistance I may be so chosen that with this current flow therethrough the potential drop across it will be precisely that value required to balance out the electromotive forces produced by the thermocouple 3. The potentiometric network 6 will then be precisely balanced. This equality in potential across resistances H2 and H4 is obtained when the triodes I08 and I09 are equally conductive or in other words, when the signal impressed on the input circuits of the triodes I08 and I09 from the transformer secondary winding I01 is zero.

Upon a change in the electromotive force produced by the thermocouple 3 from the value of precise potentiometric balance, for example, upon an increase in thermocouple electromotive force, the potentiometric network 6 will become unbalanced and-the unbalanced potentials in the potentiometric network will be translated by the interrupter 8 and the transformer I into an alternating voltage which is of the proper phase to drive the control electrode of triode I09 positive during the half cycle of the alternating voltage supply when the anode of the triode I09 is also positive and to drive the control electrode of triode I08negative when the anode of triode I08 is positive. The current in the output circuit of the triode I09 is then rendered greater than the current in the output circuit of the triode I08, and consequently, the potential drop across resistance II4 then predominates over the potential drop across resistance II2. Since the potential drop across resistance H4 is in the direction tending to drive the control electrode of valve IIB positive with respect to the potential of its associated cathode, the current flow in the output circuit of valve IIB will be increased to thereby effect an increase in the potential drop across the resistance 1. This change in potential drop across resistance I is in the proper direction to tend to, restore the state of balance of the potentiometric network 8. Some slight unbalance of the potentiometric network 8 is required to maintain the increased current in the output circuit of the valve Iii, however, and therefore, the potentiometric network 6 will not be precisely balanced, but for all practical purposes will be substantially rebalanced since the degree of unbalance required to maintain the new value of output current from valve III; is insigniflcant.

Upon a decrease in the electromotive force developed by the thermocouple 3 from the value of precisepotentiometric balance the potentiometric network 6 will be unbalanced in the opposite sense and the unbalanced potentiometric currents will be translated by the interrupter 8 and the transformer I0 into an alternating current of the opposite phase and this alternating current of opposite phase will operate to render the triode I08 more conductive than triode I09. Accordingly, the potential drop across the resistance II2 will be rendered greater than that across resistance II4. This will tend'to cause the application of a negative potential to the control electrode of valve IIS and thereby a decrease in th output current flow from the valve I I 8. This change in current flow from the valve H6, and thereby through the resistance 1, is in the proper direction to eifect a reduction in the unbalanced current flows in the potentiometric network 8.

The current flow through the resistance I required to balance out the thermocouple electromotive force will be indicated by the meters 51 and 58. When these meters are calibrated in terms of temperature, they will directly provide a measure of the temperature to which the thermocouple is subjected.

A condenser I03 is provided between the lower nd of the transformer secondary winding Ill and ground for the purpose of eliminating undesired stray alternating currents which may be extraneously introduced into the potentiometric network 6. By providing condenser I 08' such induced stray alternating currents are by-passed to ground and their effect on the operation of the measuring apparatus is thus materially reduced, if not wholly eliminated.

In order to compensate the measuring apparatus described for variations in the ambient temperature to which the cold junctions of the thermocouple 3 may be subjected, means have been provided in the form of a bridge network I2I which has its equalizing or output terminals con nected in the potentiometric network in series with the thermocouple I and the resistance I. The bridge network I2I includes resistances I22 and I 28 in opposite arms thereof and resistances I24 and I2! in the remaining arms thereof. The resistances I22 and I28 are preferably of the type having a negligible temperature coefflcient of resistance, that is the resistance values of these elements remain substantially constant notwithstanding variations in temperature to which these resistances are subjected. On the other hand, resistances I24 and I 25 are so chosen as to have an appreciable temperature coei'iicient of resistance. That is to say, the resistance values of the resistances I24 and I20 vary in correspondence with the variations in ambient temperature to which these resistances are sub- Jected. Th resistances I24 and I2! are preferably disposed closely adjacent the cold Junctions of the thermocouple 3 so that the cold junctions of the thermocoupleand the resistances I24 and I28 will be maintained at the same ambient temperature.

Energizing current is supplied the bridge network I2I from the supply lines L and U through a suitable transformer I 20' and a full wave rectifler I28 which may be of the copper oxide type. Resistances I28 and I29 may desirably be connected in the energizing circuit to the bridge network I2I from the output terminals of the rectifier I28. A condenser I2! is provided across the output terminals of the rectifier I28 for smoothing out the ripple in the rectifier output voltage and may be connected, as-shown, between one energizing terminal of bridge I2I and a point intermediate resistances I28 and I29.

The resistance values for the resistances I22 and I 22, I24 and I28 of the bridge network I2I are so chosen that when the ambient temperatur to which the cold junctions of the thermocouple 8 and the resistances I24 and I2! are sub the thermocouple 8 will be decreased because.

in a corresponding change in the potential across a the temperature difierence between the thermocouple hot and cold Junctions will then decrease.

11. this decreas in thermocouple electromotive tive force is of the proper polarity to oppose the change in thermocouple electromotive force developed by the thermocouple 3, and when the bridge network is properly designed will be the precise value to exactly compensate for the change in thermocouple electromotive force produced by the ambient temperature change.

On the other hand, upon a decrease in the ambient temperature, theelectromotive force produced by the thermocouple 3 will be correspondingly increased. In this case an electromotive force will appear between the equalizing or balancing terminals of the bridge network I2I which is of the proper polarity to again oppose the change in thermocouple electromotive force and will be of the precise value to exactly compensate for the change in thermocouple electromotive force. 7

The arrangement described bridge network I2I also operates to effect another advantageous function,. namely, compensation for the efiect of variations in the voltage of the supply lines L and L upon the operation of the measuring apparatus. Variations in the voltage provided by supply conductors L and L operate to effect corresponding variations in the potential drop across resistance 1 as will be readily recognized by those skilled in the art. Such variations in potential drop across resistance I, if not compensated for, would produce undesired errors in measurement obtained.

In accordance with the present invention, such variations in the potential drop across resistance I are compensated for by means of the bridge network I2I as a consequence of the action of the resistances I24 and I25 changing in value with variation in the current flow therethrough. That is to say, upon a change in the voltage of the supply lines L and L' the current supplied including the resistance I, an unbalanced potential will a pear between the equalizing or balancing terminals of the bridge network of the proper polarity and magnitude to exactly compensate for the variation in potential across resistance I produced by such line voltage variation. v

, Such compensation for variations in the am-- bient temperature to which the cold junctions of the thermocouple 3 are subjectedand such compensation" for variations in line voltage of the alternating current supply lines may be obtained by the same bridge network I2I by properly choosing the values and characteristics oi the resistance elements I22, I23, I24 and I25.

It will be understood that, if desired, the resistances I22 and I23 0! bridge I2I may becomposed of material having an appreciable temperature coefllcient of resistance and may be disposed closely adjacent the cold junctions of the thermocouple 3 to maintain the resistances I22 and I23 at the same ambient temperature as the cold junctions also. By providing resistances I22 and I23 having a negative temperature cobient temperature may be accomplished with the bridge operating on a smaller current fiow from the rectifier I26. I22 and I23 maybe composed of material such as carborundum, and by way of example may be resistances of the type known and sold commercially under the trade name Thermistors by the to the energizing terminals of the bridge network I2l will be changed to a corresponding extent. This change in current fiow through the resistances I24 and I25 operates to change the operating temperature of these resistances and thereby operate to eifect a change in the resistances thereof. The resistances I24 and I25 are preferably of the type in which the relationship between current flow and temperature is not a straight line relationship, that is, the voltage drops through the resistances I22 and I23 bear a straight line relationship to the current flow therethrough, but no such straight line relation obtains for the resistances I24 and I25. As the current flow through the latter resistances is increased the temperature of the latter is increased and because of the temperature coeflicient of resistance exhibited by these resistances the voltage drop across these resistances also increases. The resistances I24 and I25, for example, may be composed of nickel.

Thus, upon change in the voltage of the alternating current supply lines L and L'-' resulting Western Electric Company.

' When the gain of the electronic amplifier l approaches infinity compensation for the undesired effects of variations in the voltage of the supply lines L and L is not required because of the inherent stability of the measuring apparatus under thiscircumstance. It may be necessary to provide additional stages of amplification in the amplifier I to obtain a gain which approaches infinity. With such a high gain amplifier I only a very small unbalanced electromotive force is required in the potentiometric network to vary the conductivity of the electronic valve IIB throughout its entire range of variation. Consequently, upon a change in the potential drop across resistance 1 due to change in the voltage of the supply lines L and L the resulting unbalanced electromotive force produced in the potentiometric network operates to produce a corrective adjustment of the conductivity of valve IIG to restore the current fiow through resistance 1 and thereby the potential drop thereacross substantially to its original value. Only a very minute unbalanced potential in the potentiometric network 6 is required to maintain the electronic valve H6 at its new state of conductivity, and therefore, for all practical purposes the current flow through the resistance 1 and through the meters 51 and 58 is restored to the existing value prior to the line voltage variation.

When an electronic amplifier I the gain of which approaches infinity is employed, the transformer I26 which is provided for energizing the rectifier I28 from the supply lines L and L may be a constant voltage transformer for example of the type manufactured and sold commercially by the Solar. Electric Company and disclosed in To this end the resistances U. S. Patent 2,143,745 which was issued on January 10, 1939, to Joseph C. Sola or of the type disclosed in application Serial Number 349,625 filed on August 2, 1940, by Earl A. Keeler. The use of a constant voltage transformer in lieu of the transformer I results in the maintenance of a constant energizing voltage on the rectifier I26 irrespective of variations in the voltage of the alternating current supply conductors L and L over a wide range and thereby in the maintenance of a constant direct current voltage on the bridge circuit I2l. Accordingly, the compensation for variations in the cold junction temperature of the thermocouple'3 which is produced by the bridge network I2! will be effective to a high degree of accuracy and will be independent of variations in the voltage of the supply statutes, I have illustrated and described the best form of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in th appended claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure \by Letters Patent is:

1. Measuring apparatus including a first unidirectional E. M. F. which is variable in magnitude and which varies in magnitude in accordance with the variations in magnitude of a second unidirectional plurality of variable conditions, a circuit connection to oppose said first and second unidirectional E. M. F.s to derive a differential E. M. F., means responsive to said differential E. M. F. to vary the magnitude of said first unidirectional E. M. F. to reduce said differential E. M. F., and means in said circuit connection to compensate for variations in said firstunidirectional E. M. F. due to variations in said first mentioned variable condition and to compensate for variations in said second unidirectional E. M. F. due to variations in at least one of said variable conditions.

2. Measuring apparatus including a source of energizing voltage, means to derive from said source a first unidirectional E. M. F. of magnitude which varies in accordance with the variations in magnitude of said energizing voltage, a second unidirectional E. M. F. which varies in magnitude in accordance with the variations in a plurality of variable conditions, a circuit connection to oppose said first and second unidirectional E. M.'F.s to derive a differential E. M. F., means responsive to said difierential E. M. F. to control said first mentioned means to vary said first unidirectional E. M. F. to reduce said difierential E. M. F., and means in said circuit connection to compensate for variations in said first unidirectional E. M. F. due to the variations in voltage of said energizing source and to compensate for variations in said second unidirectional E. M. F. due to variations in at least one of said variable conditions.

3. Measuring apparatus including a first unidirectional E. M. F. which is variable in magnitude and which varies in magnitude inaccordance with the variations in magnitude of a variable condition, a thermocouple subjected to a variable temperature condition to be measured and producing a. unidirectional E. M. F. varying in accordance with the variations in said temperature condition, a circuit connection to oppose said first and second unidirectional E M. F.s to derive a differential E. M. F., means responsive to said difi'erential unidirectional E. M. F. to reduce said differential E. M. F., and means in said circuit connection responsive to the variations in said first mentioned variable condition to compensate for variations in said first unidirectional E. M. I". produced thereby and responsive to variations in the temperature of the cold Junctions of said thermocouple to compensate for variations in the second unidirectional E. M. F. produced by variations in the thermocouple cold Junction temperature.

4. Measuring apparatus including a source of energizing voltage, means to derive from said source a first unidirectional E. M. F. of magnitude which varies in accordance with the variations in magnitude of said energizing voltage, a second unidirectional E. M. l". which varies in magnitude in accordance with the VBI'lfltluDS in a plurality of variable conditions, a circuit connection to oppose said first and second unidirectional E. M. F.s to derive a diiferential E. M. F., means responsive to said difi'erential E. M. F. to control said first mentioned means to vary said differential E. M. F., and a bridge circuit connected in said circuit connection to introduce a compensating unidirectional E. l M. F. therein, said bridge circuit being energized by said energizing voltage and including means which is responsive to variations in said energizing voltage to compensate for variations in said first unidirectional E. M. F. due to variations in said energizing voltage and is responsive to variations in at least one of said variable conditions to compensate for variations in said second unidirectional E. M. F. due to variations in said one variable condition.

5. The combination of claim 4 wherein said source of energizing voltage is a source of alternating voltage and said first mentioned means includes rectifying means.

6. The combination of claim 4 wherein said source of energizing voltage is a source of alternating voltage, said first mentioned means includes electronic valve rectifying means, said second mentioned means includes means to translate said differential E. M. F. into an aitemating E. M. F., means to amplify said alternating E. M. F. and to apply said amplified quantity to control said electronic valve rectifying means, wherein rectifying means are interposed between said bridge circuit and said energizing voltage whereby said bridge circuit is energized with unidirectional current, and wherein the responsive means of said bridge circuit include a resistance element having a temperature, coefficient of resistance.

7. Measuring apparatus including a source of energizing voltage, means to derive from said source a first unidirectional E. M. F of magnitude which varies in accordance with the variations in magnitude of said energizing voltage, a thermocouple subjected to a variable temperature condition to be measured and producing a unidirectional E. M. F. varying in accordance with the variations in said temperature condition, a circuit connection to oppose said first and second unidirectional E.- M. F.s to derive a difierene tial E. M. F., means responsive to said differential E. M. F. to vary the multitude of said first unidirectional E. M. I". to reduce said differentlal E. M. F., and a bridge circuit connected in said circuit connection to introduce a compensating unidirectional E. M. F. therein, said bridge circuit being energized by said energizing voltage and including means which is responsive to variations in said energizing voltage to compensate forvariations in said first unidirectional E. M. F. due to variations in said energizing voltage and is responsive to variations in the cold junction temperature of said thermocouple to compensate for variations in said second unidirectional E. M. F. due to variations in the thermocouple cold junction temperature.

8. The combination of claim 7 wherein said source of energizing voltage is a source of alternating voltage, said first mentioned means includes electronic valve rectifying means, said second mentioned means includes means to translate said diflerential E. M. F. into an alternating E. M. F., electronic amplifying means to amplify said alternating E. M. I". and to apply said amplified quantity to control said electronic valve rectifying means, wherein rectifying means are interposed between said bridge circuit and said energizing voltage whereby said bridge circuit is energized with unidirectional current, and wherein the responsive means of said bridge circuit include a resistance element having a temperature coefllcient of resistance.

9. Measuring apparatus including a first unidirectional E. M. F. which is variable in magnitude and which varies in magnitude in accordance with the variations in magnitude of a variable condition, a second unidirectional E. M. F. to be measured, a circuit connection to oppose said first and second unidirectional E. M. F.s to derive a differential E. M. F., means responsive to said differential E. M. F. to vary the magnitude of said first mentioned unidirectional E. M. F. to reduce said diii'erential E. M. F., and means in said circuit connection to compensate for variations in said first unidirectional E. M. F. due to variations in said variable condition.

10. Measuring apparatus including a source of energizing voltage, means to derive from said source a first unidirectional E. M. F. of magnitude which varies in accordance with the variations in magnitude of said energizing voltage, a second unidirectional E. M. F. to be measured, a circuit connection to oppose said first and second unidirectional E. M. F.s to derive a differential E. M. F., means responsive to said differential E. M. F. to control said first mentioned means to vary said first unidirectional E. M. F.

to reduce said differential E. M. F., and meansin said circuit connection to compensate for varia-- tions in said first unidirectional E. M. F. due to variations in said energizing voltage.

11. Measuring apparatus including a source of energizing voltage, means to derive from said source a first unidirectional E. M. F. of magnitude which varies in accordance with said energizing voltage, a thermocouple subjected to a variable temperature condition to be measured and producing a unidirectional E. M. F. varying in accordance with the variations in said temperature condition, a circuit connection to oppose said first and second unidirectional E. M. F.s to derive a differential E. M. F., means responsive to said differential E. M. F. to vary the magnitude of said first unidirectional E. M. F. to reduce said diflerential E. M. F., and a bridge circuit having its equalizing terminals connected in said circuit connection and its energizing terminals connected in circuit with said energizin voltage source to introduce an E. M. F. into said circuit connection to compensate for variations in said first unidirectional E. M. F. due to variations in said energizing voltage.

12. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. including a second variable unidirectional E. M. F.

which varies in magnitude in accordance with' the variations in magnitude of a variable condition. a circuit connection to oppose said E. M. F.s to derive a differential E. M. F., physically stationary means controlled directly and electrically by said differential E. M. F. to regulate the magnitude of said second mentioned E. M. F. to reduce the magnitude of said differential E. M. F., and means in said circuit connection to compensate for variations in said second mentioned E. M. F. due to variations in said variable condition.

13. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. including a second variable unidirectional E. M. P. which varies in magnitude in accordance with the variations in magnitude of a variable condition, a circuit connection to oppose said E. M. F.s

to derive a difierential E. M. F., means including intermittently operating circuit interrupting means connected in said circuit connection to convert the differential E. M. F. into a fluctuating E. M. F., physically stationary means controlled by said fiuctuating E. M. F. to regulate the magnitude of said second mentioned E. M. F. to reduce the magnitude of said diiferential E. M. F., and means in said circuit connection to compensate for variations in said second mentioned E. M. F. due to variations in said variable condition.

. variations in magnitude of said energizing voltage, a circuit connection to oppose said E. M. F's. to derive a differential E. M. F.,, physically sta tionary means controlled directly and electrically by said differential E. M. F. to regulate the magnitude of said second mentioned E. M. F. to reduce the magnitude of said diiferential E. M.

and a bridge circuit having its equalizing terminals connected in said circuit connection and its energizing terminals connected in circuit with said energizing voltage source to introduce an E. M. F. in said circuit connection to compensate for variations in said second mentioned E. M. F.

due to variations in said energizing voltage.

15. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. which E. M. F. varies in magnitude in accordance with the variations in magnitude of a plurality of variable condition, including a unidirectional E. M. F. of known magnitude, a circuit connection to oppose said E. M. F's. to derive a diil'erential E. M. F., physically stationary means controlled directly and electrically by said differential E. M. F. to regulate the magnitude of said known E. M. F. to reduce the magnitude of said differential E. M. F., and means in said circuit connection to compensate for variations in said first mentioned unidirectional E. M. F. due to variations in at least variations in magnitude of a plurality of variable conditions. including a. unidirectional l"-. M. F. of known magnitude, a circuit connection to oppose said E. M. F's. to derive a diflerential E. M. F., means including intermittently operating circuit interrupting means connected in said circuit connection to convert the diflerential E. M. F. into a fluctuating E. M. F., and physically stationary means controlled by said fluctuating potential to regulate the magnitude of said known E. M. F. to reduce the magnitude of said differential E. M. F., and means in said circuit connection to compensate for variations in said first mentioned unidirectional E. M. F. due to variations in at least one 01' said plurality of variable conditions.

17. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. which E. M. F. varies in magnitude in accordance with the variations in magnitude of a plurality of variable conditions, including a unidirectional E. M. F. of known magnitude, a circuit connection to oppose said E. M. F's. to derive a difl'erential E. M. F., physically stationary means controlled directly and electrically by said differential E. M. F. to regulate the magnitude oi. said known E. M. F. to reduce the magnitude of said differential E. M. F., and a bridge circuit means having its equalizing terminals connected in said circuit connection to compensate for variations in said first mentioned unidirectional E. M. F. due to variations in at least one of said plurality of variable conditions.

18. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. produced by a thermocouple subjected to a variable temperature condition to be measured, including a unidirectional E. M. F. of known magnitude, a circuit connection to oppose said E. M. F5. to derive a diflerential E. M. F., physically stationary means controlled directly and electrically by said differential E. M. F. to regulate the magnitude of said known E. M. F. to reduce the magnitude of said diflerential E. M. F., and a bridge circuit connected in said circuit connection to introduce a compensating unidirectional E. M. F. therein, said bridge circuit including means which is responsive to variations in the cold junction temperature of said thermocouple to compensate for variations in said thermocouple E. M. F. due to variations in the thermocouple cold junction temperature.

19. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. which E. M. F. varies in magnitude in accordance with the variations in magnitude oi!v a plurality of variable conditions, including a source of energizing voltage, means to derive from said source a second unidirectional E. M. F. of magnitude which varies in accordance with the variations in magnitude of said energizing voltage, a circuit connection to oppose said E. M. F's. to derive a differential E. M. F., physically stationary means controlled directly and electrically by said diiierential E. M. F. to regulate the magnitude of said second mentioned E. M. F. to reduce the magnitude of said differential E. M. F., and a bridge circuit having its equalizing terminals connected in said circuit connection and its energizing terminals connected in circuit with said energizing voltage source to introduce an E. M. F. in said circuit connection to compensate for variations in said first mentioned E. M. F. due to variations in at least one of said plurality of variable conditions and to compensate for variations in said second mentioned E. M. F. due to variations in the magnitude of said energizing voltage.

20. Sell balancing apparatus for measuring the magnitude of a unidirectional E. M. I". which E. M. I". varies in magnitude in accordance with the variations in magnitude of a plurality of variable conditions, including a source or energizing voltage, means to derive irom said source a second unidirectional E. M. F. of magnitude which varies in accordance with the variations in magnitude of said energizing voltage, a circuit connection to oppose said E. M. F.'s to derive a differential E. M. F., mean including intermittently operating circuit interrupting means connected in said circuit connection to convert the differential E. M. 1'. into a fluctuating E. M. F.. and physically stationary means controlled by said fluctuating E. M. F. to regulate the magnitude of said second mentioned E. M. F. to reduce the magnitude of said differential E. M. F., and a bridge circuit having its equalizing terminals connected in said circuit connection and its energizing terminals connected in circuit with said energizing voltage source to introduce an E. M. F. in said circuit connection to compensate for variations in said first mentioned E. M. F. due to variations in at least one of said plurality of variable conditions and to compensate for variations in said second mentioned E. M. F. due to variations in the magnitude of said energizing voltage.

21. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. produced by a thermocouple subjected to a variable temperature to be measured including a source of energizing voltage, means to derive from said source a second unidirectional E. M. F. or magnitude which varies in accordance -with the vari-' ations in magnitude of said energizing voltage, a circuit connection to oppose said E. M. F.s to derive a differential E. M. F., Physically stationary means controlled directly and electrically by said differential E. M. F. to regulate the magnitude of said second mentioned E. M. F. to reduce the magnitude of said differential E. M. F., and a bridge circuit having its equalizing terminals connected in said circuit connection and its energizing terminals connected in circuit with said energizing voltage source to introduc an E. M. F. in said circuit connection to compensate for variation in said first mentioned E. M. F. due to variations in the temperature of the cold junc tions of said thermocouple and to compensate for variations in said second mentioned E. M. F. due to variations in the magnitude of said energizing voltage, said bridge circuit having means connected therein which is responsive to the cold junction temperature of said thermocouple and i responsive to the magnitude oi! said energizing voltage.

22. Measuring apparatus including a unidirectional E. M. F. which is variable in magnitude and which varies in magnitude in accordance with the variations in magnitude of a variable condition, a unidirectional E. M. F. to be measured, a circuit connection to oppose said E. M. F.'s derive a diflerential E. M. F., means to vary said first mentioned E. M. F. to reduce said difi'erential E. M. F. to a minimum value, means to measure the magnitude 0! said first mentioned E. M. F., and means in said circuit connection to compensate for variations in said first mentioned unidirectional E. M. F. due to variations in said variable condition.

23. Measuring apparatus including a source of energizing voltage, means to derive from said source a unidirectional E. M. F. of magnitude which varies in accordance with the variations in magnitude of said energizing voltage, a unidirectional E. M. F. to be measured, a circuit connection to oppose said E. M. F.s to derive a differential E. M. F., means to vary said first mentioned E. M. F. to reduce said differential E. M. F. to a minimum value, mean to. measure said first mentioned E. M. F., and a bridge circuit having its equalizing terminals connected in said circuit connection and its energizing terminals connected in circuit with said source to introduce an E. M. F. in said circuit connection to compensate for variations in said first mentioned E. M. F. due to variations in the magnitude of said energizing voltage, said bridge circuit having means connected therein responsive to the magnitude of said energizing vdltage and operative to control the state of balance of said bridge circuit.

24. Measuring apparatus including a source of energizing voltage, means to derive from said source a unidirectional E. M. F. of magnitude which varies in accordance with the variations in magnitude of said energizing voltage, a unidirectional E. M. F. to be measured which varies in magnitude in accordance with the variations in magnitude of a plurality of variable conditions. a circuit connection to oppose said E. M. F.s to derive a differential E. M. F., means to vary said first mentioned E. M. F. to reduce said diiferential .E. M. F. to a minimum value, means to measure said first mentioned E. M. F., and means connected in said circuit connection to compensate for variations in said first mentioned E. M. F. due

to variations in said energizing voltage and to compensate for variations in said second mentioned E. M. F. due to variations in at least one of said variable conditions.

25. Measuring apparatus including a source of energizing voltage, means to derive from said source a unidirectional E. M. F. of magnitude which varies in accordance with thevariations in magnitude of said energizing voltage, a thermocouple subjected to a variable temperature condition to be measured and producing a unidirectional E. M. F. varying in accordance with the variations in the temperature condition, a circuit connection to oppose said E. M. Fs. to derive a differential E. M. F., means to vary said first mentioned E. M. F. to reduce said" differential E. M. F. to a minimum value, means tomeasure first mentioned E. M. F., and a bridge circuit having its equalizing terminals connected in said circuit connection and its energizing terminals connected in circuit with said energizing voltage source to introduce a compensating E. M. F. into said circuit connection. said bridge circuit having means connected therein responsive to the magnitude of said energizing voltage and responsive to the cold junction temperature of said thermocouple and operative to control the state of balance of said bridge circuit in such manner that the compensating E. M. F. introduced into said circuit connection is effective to compensate for the variations in said first mentioned E. M. F. due to variations in magnitude of said energizing volt- Y age and to compensate for the variations in said second mentioned E. M. F. due to variations in the cold junction temperatures of said thermocouple.

26. Measuring apparatus including a first unidirectional E. M. F. which is variable in magnitude and which varies in magnitude in accordance with the variations in magnitude of a variable condition, a second unidirectional E. M. F. to be measured and which varies in magnitude in accordance with the variations in a plurality of variable conditions, a circuit connection to oppose said first and, second unidirectional E. M. F.s to derive a differential E. M. F., means responsive to said differential E. M. F. to vary the magnitude of said first mentioned unidirectional E. M; F. to reduce said differential M. F., said last mentioned means including means to compensate for variations in said first unidirectional E. M. F. due to variations in said first mentioned variable condition, and means in said circuit connection to compensate for variations in said second unidirectional E. M. F. due to variations in at least one of said variable conditions.

2'1. Measuring apparatus including a first unidirectional E. M. F. which is variable in magnitude and which varies in magnitude in accordance with the variations in magnitude of a variable condition, a second unidirectional E. M. F. to be measured and which varies in magnitude in accordance with th variations in a plurality of variable conditions, a circuit connection to oppose said first and second unidirectional E. M. Fs. to

derive a differential E. M. F., means including,

intermittently operating means connected in said circuit. connection to convert the differential E. into a fluctuating E. M. F.. means including a high gain electronic amplifier to amplify said fluctuating E. M. F. and to utilize the amplifled quantity to vary the magnitude of said first mentioned unidirectional E. M. F.'to reduce said diiferential E. M. F. and which operates to compensate for variations in said first unidirectional due to variationsin said first mentioned variable condition, and means in said circuit connection to compensate for variations in said secondunidirectional E. M. F. due to variations in at least one of said variable conditions.

28. Measuring apparatus including a first unidirectional E. M. F. which is variable in magnitude and which varies in magnitude n accordance with the variations in magnitude of a variable condition; a second unidirectional E. M. F. to be measured, means to oppose said first and second unidirectional E. M. Fs. to derive a differential E. M. F., and means responsive to said differential E. M. F. to vary the magnitude of said first mentioned unidirectional E. M. F. to reduce said dififerential E. M. F., said last mentioned means including means to compensate for variations in said first unidirectional E. M. F. due to variations in said variable condition.

29. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. including a second variable unidirectional E. M. F. which varies in magnitude in accordance with the variations in magnitude of a variable condition, a circuit connection to oppose said E. M. Fs. to derive a differential E. M. F., and physically stationary means controlled directly and electrically by said differential E. M. F. to regulate the magnitude of said second mentioned E: M. F. to reduce the magnitude of said difierenti'al E. M. F., said last mentioned means including means to compensate for variations in said second mentioned E. M. F. due to variations in said variable condition.

30. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. including a second variable unidirectional E. M. F. which varies in magnitude in accordance with the variations in magnitude of a variable condition, a circuit connection to oppose said E. M. Fs. to derive a differential E. M. F., means including intermittently operating circuit interrupting means connected in said circuit connection to convert the differential E. M. F. into a fluctuating E. M. F., and physically stationary means including a high gain electronic amplifier for amplifying said fluctuating E. M. F. to regulate the magnitude of said second mentioned E. M. F. to reduce the magnitude of said differential E. M. F.

31. Self balancing apparatus for measuring the magnitude of a unidirectional E. M. F. including a source of energizing voltage, means to derive from said source a second unidirectional E. M. F. of magnitude which varies in accordance with the variations in magnitude of said energizing vo1tage, a circuit connection to oppose said E. M. F's. to derive a differential E. M. F., means including intermittently operating circuit interrupting means connected in said circuit connection to convert the diiierential E. M. F. into a fluctuating E. M. F., and means including an electronic amplifier the gain of which approaches infinity to amplify said fluctuating E. M. F. and to utilize said amplified quantity to control said first mentioned means to regulate the magnitude of said second mentioned E. M. F. to reduce the magnitude of said diflerential E. M. F.

HARRY S. JONES. 

